The present invention relates to the forming of drain holes from a cased well and more particularly relates to the drilling of substantially horizontal drain holes into a subterranean formation from a previously completed, substantially vertical cased wellbore to thereby effectively extend the production life of said formation.
The use of drain holes to increase the production from hydrocarbon bearing formations has long been known. These drain holes are formed by drilling curved boreholes into the producing formation from at or near the bottom of a main, vertical wellbore before the well is cased and initially completed. In one known technique, a deflecting tool, e.g. special whipstock, is supported on an anchor which digs into the bottom of the main bore. A specially designed, flexible drill string is then lowered into engagement with the deflecting tool to drill a drain hole substantially horizontal from the main bore. For a more complete description of this technique, see U.S. Pat. Nos. 3,349,845 and 3,398,804. In the technique described above, the drain holes are drilled from an open, uncased borehole.
With the advent of recent, worldwide shortages of hydrocarbons, the need to extend the production life of older wells is now more important than ever. One way to increase the production life of several of these older wells is to drill drain holes into a producing formation to increase the effective permeability of the formation therby improving the communication between distant points in the formation and the main wellbore through which the hydrocarbons can flow. However, normally these older wells, when originally completed, are cased with heavy walled, steel casing which is cemented in place throughout the length of the main borehole which, as far as known, has heretofore interferred with the drilling of substantially horizontal drain holes from the main wellbore.